19 IS] LUMIXESCEXCE OF ZlKCOXS 



iO 



Chem. Soc. 89: 1568). On heating, the mineral gives off both helium 

 and argon. Twenty per cent of the helium is accounted for by the 

 uranium present. Antropofi" (Z. Elektrochem. 14: 585) claims to 

 have secured argon on heating native zirconia (baddeleyite) from 

 Brazil. These observations should be most carefully tested by fur- 

 ther investigations, as they open far-reaching theoretical questions. 



This radio-activity of zircons has produced in the enclosing biotite, 

 iolite, etc., the usual pleochroitic lialos observed in connection with 

 other radio-active minerals. 



Strutt, as cited, makes use of the radio-activity of zircons for de- 

 termining the age of geologic formations. Zircon crystals in plutonic 

 rocks are opaque, those in basalt and lavas are transparent and show 

 signs of incipient fusion. The transparent crystals are thermo- 

 luminescent, giving out a phosphorescent glow and losing color when 

 moderately heated. This property of thermo-luminescence has been 

 noted in a number of minerals, as certain fluorites, etc. The glow is 

 not repeated on a second heating if once heated until it disappears. 

 Strutt found that it can be restored, and also the color, by exposure 

 to the action of radium salts. This alternation can be repeated a])par- 

 ently indefinitely. Fluorspars act in the same way. Of course it 

 is well known that the silicates in ordinary glass containers of radium 

 compounds are deeply colored after sufficient exposure. Opaque 

 zircons are not thermo-luminescent nor made so by exposure to 

 radium. I^or are they decolorized by moderate heating. If kept in 

 incited basalt lor twenty-four hours they become white though not 

 transparent, and then on exposure to radium emanations they acquire 

 a red-brown color like the hyacinth and are thermo-hnnincscent. 

 This treatment, however, does not make them transj)arent. 



It is stated by Demarcay (('oin|)t. Kcnd. 104; ll-'!) that /irciuis 

 lose their color when heated in a stream of carbon tetrachloride. 

 Only imperfect success was obtained on subjecting o|)a(|ue zircons to 

 this treatment. They were practically whiteiu'd and some small 

 splinters seemed to be transparent. 



ClIAI'KI. HlIL. N. C. 



